Clutches for controlling power are classified into a wet clutch, a dry clutch, a conical clutch, a magnetic clutch, a dog tooth clutch, etc.
Recently, vehicles with coaxial dual clutches were introduced. Dual clutch transmissions (DCT) refer to a transmission whereby two clutches are disposed in the front of the transmission and selectively and successively operated so that shift time can be markedly reduced. Such dual clutch transmissions are an evolution of automated manual transmissions and are presently in the limelight because they have as high economic feasibility as manual transmissions, can reduce power loss and enhance power efficiency, and can realize rapid shifting and thus provide a sporty driving experience.
Well known dual clutch transmissions are advantageous in that the performance thereof is excellent whereby, when a vehicle accelerates, a speed gear of a higher stage than a current speed gear is pre-selected to make shifting to a higher gear stage rapidly, and when the vehicle decelerates, a speed gear of a lower stage than a current speed gear is pre-selected and thus engine braking occurs. However, when the vehicle is moving at a constant speed or repeats acceleration and deceleration, it is difficult to control and determine whether having to pre-select a speed gear of a higher stage than a current speed gear or a speed gear of a lower stage than.
Furthermore, in the known dual clutch transmissions, although it is easy to shift gears from an even gear stage to an odd gear stage or from an odd gear stage to an even gear stage, it is impossible to conduct skip shifting whereby gears are shifted from an even gear stage to another even gear stage or from an odd gear stage to another odd gear stage. Such skip shifting is widely used for rapid acceleration and, particularly, requires a rapid and smooth shifting response.
Therefore, some people are of the opinion that there is need for a transmission with at least three clutches. In other words, a transmission is required wherein one or more gear stages higher and one or more gear stages lower than an engaged speed gear must always be pre-selected.
FIG. 1 is a schematic view showing the configuration of a gear train of a triple clutch transmission according to a conventional technique. FIG. 2 is a schematic view showing the configuration of a gear train of a triple clutch transmission according to another conventional technique. FIG. 1 illustrates Korean Patent Unexamined Publication No. 10-2005-0042671, entitled “TRIPLE CLUTCH TRANSMISSION”. FIG. 2 shows U.S. Pat. No. 7,552,658B2, entitled “THREE CLUTCH POWERSHIFT TRANSMISSION”. The former is configured such that three shafts are coaxially provided, wherein a first shaft that is a basic input shaft is provided, a second shaft that is a hollow shaft is provided around the outer surface of the first shaft, a third shaft that is a hollow shaft is provided around the outer surface of the second shaft, and speed gears are provided on each shaft. Furthermore, first to third clutches C1 to C3 are connected to the shafts so as to control transmission of power.
This triple clutch transmission overcomes the above-mentioned disadvantages of dual clutches and is configured such that gears one stage higher and one stage lower than a current gear stage are pre-selected, whereby smooth sequential shifting can be realized, and skip shifting can be more easily conducted.
However, it is very difficult to form the coaxial shaft structure. In addition, the production cost is increased, and repair and maintenance are difficult. Furthermore, if the number of gear stages is increased to seven or more stages, the structure of the transmission is further complicated. Moreover, it is not easy to control the transmission because of characteristics in which gear selectors (synchro devices) are operated at three positions including left, neutral, and right positions.
The latter, that is, the technique of the US patent, also pertains to a triple clutch transmission. However, unlike the above conventional technique, first to third hollow shafts 22, 28, and 30 are coaxially provided around an outer surface of a first shaft 20 that is an input shaft, and first to third hydraulic clutches 14, 16, and 18 are provided on the respective hollow shafts such that transmission of power is separately controlled.
This transmission structure can overcome the disadvantages of the dual clutch whereby sequential shifting or skip shifting can be conducted during acceleration or deceleration of the vehicle. However, as shown in the drawing, the structure of the gear train is also very complex. Thus, there are still problems of a complex manufacturing process and difficult maintenance and repair.
Furthermore, the speed gears of six gear stages are arranged in a line on the first to third hollow shafts without use of a gear selector. Therefore, it is difficult to apply this technique to a transmission with seven or more gear stages. Thus, this conventional technique cannot meet recent trends of subdividing the gear stages and increasing the number of gear stages.